The present invention relates to novel catalyst compositions and to processes for making polyolefin resins using such novel catalyst compositions, polyolefin resins, and articles made from such polyolefin resins. In particular, this disclosure relates to processes for making bimodal polyolefin resins using a novel catalyst composition comprising a bimetallic transition metal catalyst precursor and a cocatalyst. This disclosure also relates to polyolefin resins with improved properties (e.g., improved bubble stability) having bimodal molecular weight distributions and long chain branching, as well as articles made from such polyolefin resins.
Increasing the molecular weight of polyethylene (and copolymers of ethylene) generally results in enhancing tensile strength, ultimate elongation, impact strength, puncture resistance, and toughness of films. However, increasing the molecular weight of the polyethylene will usually decrease its processability. By providing a blend of a relatively high molecular weight (HMW) ethylene polymer with a relatively lower molecular weight (LMW) ethylene polymer, the desirable characteristics due to the relatively high molecular weight polymer component can be retained while, at the same time, improving processability of the blend material containing the relatively high molecular weight and low molecular weight polymer components.
To produce such blends, various alternatives are being considered in the art, including post reactor or melt blending, catalysis in a single reactor with a catalyst effective to produce the blend material, and use of multistage reactors in which different molecular weight components can be produced sequentially in each reactor.
U.S. Pat. No. 2,924,593 to Breslow discloses a process for producing high molecular weight polyethylene comprising contacting ethylene with a catalyst composition comprising a bis(cyclopentadienyl)zirconium salt and a metal alkyl compound of an alkali metal, an alkaline earth metal, or aluminum. In Example 7, the catalyst composition is formed in situ by contacting bis(cyclopentadienyl)zirconium dichloride, triethylaluminum, and ethylene in toluene.
U.S. Pat. No. 4,701,432 to Welborn, Jr. discloses a catalyst system comprising (i) a metallocene and a non-metallocene transition metal compound (i.e. a transition metal compound not containing cyclopentadienyl) supported catalyst component and (ii) a combination of an organometallic compound of a metal of Groups IA, IIA, IIB and IIIA of the Periodic Table and an alumoxane cocatalyst. The catalyst composition is disclosed as being useful for olefin polymerization, and particularly for the production of linear low, medium and high density polyethylenes and copolymers of ethylene with alpha-olefins having 3 or more carbon atoms (C3-C18), cyclic olefins, and/or diolefins having up to 18 carbon atoms.
U.S. Pat. No. 5,049,535 to Resconi, et al. discloses that the activity of a catalyst composition obtained from zirconocenes and trialkylaluminum compounds is extremely low when applied to the polymerization of ethylene and practically nil for higher olefins (column 1, lines 10-26). To increase activity, Resconi, et al. proposed the use of substituted metallocene compounds in combination with trialkylaluminum compounds.
U.S. Pat. No. 5,157,008 to Sangokoya, et al. discloses the production of hydrocarbon solvent solutions of alkylalumoxanes by mixing trimethylaluminum and a hydrocarbylaluminum compound, which compound contains at least one hydrocarbyl group having 2 or more carbon atoms, in a hydrocarbon solvent and thereafter adding water or a hydrated compound so as to form a solution of alkylaluminoxane in said solvent.
U.S. Pat. No. 5,238,892 to Chang discloses an olefin polymerization catalyst composition comprising a solid product produced by mixing and reacting a metallocene and an aluminum alkyl, for example trialkylaluminum, in a hydrocarbon solvent to form a reaction product, and thereafter adding an undehydrated support material to the reaction mixture.
U.S. Pat. No. 5,332,706 to Nowlin, et al. discloses that the metallocene catalyst must contact the alumoxane (e.g., methylalumoxane (MAO)), while the alumoxane is in solution in order for the metallocene to be activated in a fluidized-bed reactor. Moreover, the patent discloses that extensive reactor fouling results when MAO solutions are fed directly into the gas phase reactor in large enough quantities to provide this liquid contact. The fouling was found to occur because the MAO solution forms a liquid film on the interior walls of the reactor, and the catalyst is activated when it comes into contact with this liquid film, which in turn leads to the formation of a polymer coating that grows larger in size until the reactor is fouled.
U.S. Pat. No. 5,849,653 to Dall""Occo, et al. discloses catalysts for the polymerization of olefins obtained from cyclopentadienyl compounds of a transition metal, an organometallic aluminum compound, and water.
Japanese Laid-Open Patent Application (Kokai) No. 4-266891 discloses a process for producing a methylisobutylalumoxane having high activity and excellent solubility in hydrocarbons.
It would be desirable to provide a catalyst composition that is capable of producing a bimodal molecular weight distribution (MWD) polyolefin resin with improved properties (e.g., bubble stability) having a bimodal molecular weight distribution and long chain branching. Further, it would be highly desirable to provide a catalyst composition with high activity from which bimodal polyolefin resins having long chain branching can be produced, wherein the polyolefin resins do not require special post-polymerization tailoring (i.e., the polyolefin resins do not have to be treated with modifiers, such as oxygen or organic peroxides, to modify the molecular weight distribution) and yet possess excellent bubble stability.
In one embodiment, a catalyst composition is provided, wherein the catalyst composition comprises a transition metal catalyst precursor and a cocatalyst, the transition metal catalyst precursor comprising the contact product of an unsubstituted metallocene compound and an aluminum alkyl compound in a hydrocarbon solvent solution.
In an alternative embodiment, a catalyst composition is provided, wherein the catalyst composition comprises a bimetallic transition metal catalyst precursor and a cocatalyst, the bimetallic transition metal catalyst precursor comprising:
(a) the contact product of an unsubstituted metallocene compound and an aluminum alkyl compound in a hydrocarbon solvent solution; and
(b) a non-metallocene transition metal component.
Further, a process for polymerizing olefins (e.g., ethylene and/or higher olefins) is provided, wherein the process comprises contacting one or more olefins with a catalyst composition comprising a transition metal catalyst precursor and a cocatalyst, the transition metal catalyst precursor comprising the contact product of an unsubstituted metallocene compound and an aluminum alkyl compound in a hydrocarbon solvent solution.
Alternatively, another process for polymerizing olefins (e.g., ethylene and/or higher olefins) is provided, wherein the process comprises contacting one or more olefins with a catalyst composition comprising a bimetallic transition metal catalyst precursor and a cocatalyst, the bimetallic transition metal catalyst precursor comprising:
(a) the contact product of an unsubstituted metallocene compound and an aluminum alkyl compound in a hydrocarbon solvent solution; and
(b) a non-metallocene transition metal component.
In yet another embodiment, a polyolefin resin having improved bubble stability is provided, wherein the polyolefin resin has a bimodal molecular weight distribution and long chain branching.
Further, an ethylene (co)polymer is provided, wherein the ethylene (co)polymer is produced in a single reactor and has a bimodal molecular weight distribution, a flow activation energy of at least about 27 kjoule/mole, a density of from about 0.89 to about 0.965 g/cc, a melt index of from about 0.01 to about 0.2 g/10 minutes, a high load melt index (HLMI) of from about 2 to about 100 g/10 minutes, and a melt flow ratio (MFR) of from about 40 to about 300.
Other additional embodiments include various articles made from the above-described polyolefin resins.